1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission, and more particularly to a hydraulic control system for reducing shift shock in an automatic transmission during a change of gear position.
2. Description of the Prior Art
For preventing shift shock, there is known a system as proposed in U.S. Pat. No. 3,710,630 (Tokko No. Sho 49-9536 or U.S. Pat. No. 4,106,368 (Tokkai No. Sho 53-79158).
According to the system proposed in U.S. Pat. No. 3,710,630 (Tokko No. Sho 49-9536), the hydraulic pressure applied to the hydraulically actuated friction means is reduced to a low level in response to a shift or gear change command signal. The hydraulic pressure stays in the low level for a predetermined time duration. Since the low level and the predetermined time duration are constant, and variation in the engine load before or after the gear change is not taken into account at all. However, there is a drawback in that the friction means may slip excessively for non gear change operation or the decrease in hydraulic pressure may not be enough to insure a shockless gear change.
According to the system proposed by U.S. Pat. No. 4,106,368 (Tokkai No. Sho 53-79158), the output torque at the transmission output shaft is detected by a transducer and is applied to an electronic control unit which can control the gear change by regulating the hydraulic pressure in response to the detected output torque. This known system has a drawback that, under the presence of a torque variation in the engine output, torsional vibration in a driven shaft, or slipping of the wheels, the transducer can not detect the output torque accurately.